You wont find anything in the FAR's. The FAA's jurisdiction pretty much ends at the surface unless you are at an airport and even at an "airport," there are some questions about just how far the FAA's jurisdiction extends with airports ranging everything from public-use FAA grant recipient airports to uncharted private property fields (where the rights of the property owner generally take precedence) to military fields (where they have none).
The issue of landing on any piece of property ultimately comes down to the rights of the property owner. In the case of most public access roads, this would normally be the state. For the seaplane pilot, this is also normally the case of most "unowned" waterways in a state
Some states (Montana, Alaska) specifically permit the use of roads for the purpose of takeoff/landing by way of public statue.
Most states dont specifically codify it which of course raises questions in cases such as that guy in TN who recently landed on a public road for a career fair.
Some states (New Jersey) specifically restrict the use of waterways for seaplane takeoff/landing operations except at a designated seaplane base. In the case of NJ, you could technically land in international water and/or a neighboring state and taxi into NJ waters but you cant land or takeoff in NJ waters except at a seaplane base.
Other states have special restrictions for certain waterways, particularly those used for public drinking water or those with known presence (or absence) of certain invasive nuisance species such as zebra mollusks.
The real issue you can run into with the FAA is a 91.13 violation; even though the FAA has limited jurisdiction over planes on the ground and would probably be hard pressed to throw this at you just because you landed on a road, the moment you takeoff or just before touchdown you are still in the air and therefore fully under the FAA's jurisdiction... They will undoubtedly try to argue that you operated/flew in a careless or reckless manner should you chose to land on a road with traffic. It'd be much more difficult to prove if you land on an empty road. In the Trent video, they actually appear to be landing off the road in an empty field/space near the gas station.
The issue you can run into with a state is far more varied but is also limited since its not explicitly defined. Once the plane is on the ground (or water) though you could in theory become subject to the jurisdiction of the owning/administering authority which technically means, you need to abide by the all rules of the road on the highway and that includes speed limits, minimum speeds, lane blocking, etc.
EDIT: Fixed typo of 91.113 vs 91.13. Inconveniently 91.113 happens to cover a topic that would seem like it might apply but does not (right of way rules).
